In a typical medical workstation the appearance of a displayed image can be adjusted by selecting a relevant subrange of the image's grey value pixel data and converting the data in this subrange according to a specific gradation function.
An interactive method to adjust manually the width and the position of the selected subrange is commonly known as window width/level adjustment.
Such a transformation of grey value pixel data is mainly used to increase the contrast in some regions of the image or in a selected subrange of the grey value pixel data. The price to be paid is decreased contrast in other parts of the image and other ranges of the grey value pixel data.
The density of pixels and image regions is determined by the corresponding ordinate value of the gradation function. The contrast amplification of pixels and image regions on the other hand, is determined by the corresponding derivative value (i.e. the gradient) of the gradation function. As a consequence, if the shape of the gradation function is adjusted to accommodate a large subrange of grey values within a specified density interval, i.e. if the interval has to cope with wide latitude, then at the same time the contrast in that density interval will drop. On the other hand, if a density interval is assigned to only a narrow grey value subrange, then that interval will provide enhanced contrast.
If requirements with respect to density and contrast amplification are conflicting, which is often the case, then a compromise is unavoidable.
In the context of this document the term ‘first enhancement method’ will be used to refer to these kinds of methods and more specifically to the window width/level method.
In order to convert the digital image information optimally into a visible image on which the diagnosis is performed, a multi-scale image processing method (also called multiresolution image processing method) has been developed by means of which the density and contrast of an image can be modified. More specifically the density and contrast can be modified independently. In the context of this document the term ‘independently’ relates to processing methods in which modification of contrast does not have substantial influence on the density levels in the image and wherein modification of the density does not substantially influence the contrast in the image.
The above mentioned image processing method and user interface pertaining to such a method has been described extensively in U.S. Pat. No. 7,298,383, the processing being referred as multi-scale gradation processing.
In the context of this document the term ‘second enhancement method’ refers to multi-scale gradation processing methods.
In a specific embodiment of U.S. Pat. No. 7,298,383, the modification is a global modification, i.e. the change of contrast and density is applied to substantially all pixels of the displayed image.